The present invention relates to an image reader for obtaining the image data for image forming apparatus of, for example, electrophotographic system and to an image forming apparatus that is provided with the image reader.
The image reader photoelectrically converts reflected light obtained by illuminating the object being read such as a sheet-form document or book, or solid body, and so forth by the image reader sensor such as a charge-coupled device (CCD) sensor, and the like, and output the image data.
For the image reading method by the image reader, there are two systems: one a flat bed mode in which a original document is allowed to be at rest on a transparent document holding section (transparent glass) and an optical system such as a lighting device and a mirror, and the like or an image reader sensor is moved along the transparent glass and the other a sheet-through mode in which an optical system such as a mirror, and the like or image reader sensor and the illuminating device are fixed to a specified location in advance, the reading position is defined, sheet-form objects to be read are transferred successively to the reading position. Today, in many of the image readers, in order to enable the above-mentioned two reading systems, together with the transparent glass, a reading position for sheet-through that enables sheet-through is provided. Consequently, the reading position for sheet-through is generally defined to be the specified position further remote from the head end of the transparent glass which is the acceleration section used for acceleration when the optical system such as an illuminating device and a mirror, and the like or an image reader sensor travels in the flat bed mode.
Next, the basic configuration of the image reader of a flat bed mode that enables sheet-through will be described.
When the flat bed mode is adopted, the following system is used; a carriage holding the illuminating device and mirror, and the like that can illuminate the reading width, the total length of one side (width) direction of transparent glass is allowed to travel at a specified speed with a specified interval made with the transparent glass held along the reading length, the total length of the other one side (length) that intersects the reading width at right angles, and using a lens, and the like, the reflected light from the object to be read (illuminated by the illuminating device) is transmitted to the CCD sensor fixed to the position which do not interfere with the carriage travel.
On the other hand, when the image data of the sheet-form original document is obtained by the sheet-through mode, the carriage is fixed opposite to the transparent glass at the specified position where the reflected light from the reading position is able to be guided to the CCD sensor and the object to be read is conveyed to the reading position successively.
Now, in the image reader, in order to correct shading when the image of the object to be read is converted to the image data, the shading data is generated from the reflected light obtained by illuminating the white board of the specified brightness that serves as the standard for shading correction and then, shading of the read image is carried out. In addition, when the shading data is generated, the data for several lines (in the length direction) are generally balanced for each pixel (in the width direction) to prevent influences of dirt, dust, and the like (dirt, dust, and the like are read as the image).
Consequently, in general, while the carriage is being moved at the constant speed, the reflected light is read from the white board for shading correction. By the way, in the image reader as described above, the carriage is accelerated at the specified acceleration in such a manner that the carriage is moved along the transparent glass, that is, the object to be read (in such a manner that the carriage reaches to the specified speed (constant speed) when the carriage in motion reaches the glass end section) as the carriage begins to travel from the home position, the travel starting position. By the way, the home position is, in general, set at the specified location between the white board for shading correction and the reading position (for the sheet-through mode). Consequently, the time for reading the reflected light from the white board for individual lines (in the length direction) is set to the constant value in accordance with the cycle of the horizontal (reading length) synchronous signal (referred to as the “H-SYNC signal”) irrespective of the reading speed, that is, the speed at which the carriage is moved in the reading length (horizontal) direction.
However, when the home position is located in the vicinity of the white board as described above, there is a problem in that the distance between the home position and the transparent glass head end is long and the time required for first scan is long (first scan is unable to be shortened) because in the flat bed mode, the carriage is maintained temporarily to the constant speed for acquiring the shading data by the white board in the midst of acceleration from the home position and then, is accelerated to the reading speed in the section up to the transparent glass head end (there are cases in which the carriage is accelerated to the reading speed when the carriage is allowed to pass the white board).
In addition, when the image data is obtained by sheet-through in the image reader described above, there is also a problem in that the first scan time becomes still longer than in the flat bed mode because the carriage must be returned to the reading position for sheet-through after the carriage is temporarily allowed to pass the shading position opposite to the white board to obtain the shading data.
By the way, in the high-speed image reader of the flat bed mode, there is a case in which the carriage is unable to be accelerated to the specified reading speed in the section up to the transparent glass head end because the carriage is moved at a constant speed in the section opposite to the white board in order to obtain the shading data, and as a result, a problem in that the size of the image reader becomes large arises.